In its ruling, the authority said a conference centre of national and international importance should instead be built in a city or town.

This is where there would be public transport, cultural facilities and a range of accommodation.

The end-of-the-line rebuff means Mr Mansfield can now be legally obliged to remove the structure.

South Dublin Co Council said last night that Bord Pleanala was the final arbiter in planning and they would abide by the decision.

The enforcement issue would now be adjudicated in the light of the board's decision. However, it is certain the council will order Mr Mansfield to take down the structures already erected.

Asked if he would now have to take down the centre, a defiant Mr Mansfield told the Irish Independent: "I doubt it. There are plenty of things we can do."

The long-running controversy took a new turn yesterday when Bord Pleanala ruled the conference centre cannot stay up as it does not have planning permission. The convention centre was to have accommodated up to 6,000 people.

The council had granted permission to Mansfield's company, HSS Ltd, to retain the steel frame and a partial roof of the centre, which were built without planning approval. His company was fined ?1,750 and ordered to pay costs of almost ?30,000 to the council after Tallaght District Court found it had failed to comply with an enforcement order by the council to stop work.

This is the second time Bord Pleanala has ruled on the issue. It overturned the original planning permission in 2004. However, a vote last November by councillors backing completion of the centre appeared to have cleared the way for Mr Mansfield to make another application to continue and retain the development.

To get around the zoning planning difficulties, councillors put an objective into the county development plan to facilitate development of a conference centre on the site.

Zoning was one of the main reasons why Bord Pleanala rejected it first time round.

Yesterday, the planning application was shot down again by Bord Pleanala.

This means the building is unauthorised and must be taken down.

End

Planning sources said: "This would appear to be the end of the line. The council will have to act."

The appeal was brought by Mr Mansfield's company, HSS, and by An Taisce and the Point Exhibition Company.

Harry Crosbie, the Point owner, is involved in a bid with Treasury Holdings and Iarnrod Eireann to buy a 32-acre site to develop a national conference centre at Spencer Dock, Dublin.

The Mansfield convention centre involved a three-storey building, with a main hall including seating for up to 6,000 people and 675 car parking spaces.

Refusing permission yesterday, Bord Pleanala said that a major facility on such a scale "should be located in a core commercial area such as a city centre or major town centre".

This was in order to ensure there was "a range of public transport options and full access to supporting facilities - recreational, accommodation and cultural".

The board said it took the decision notwithstanding the zoning objective set by the council.

"Having regard to the location of the site outside any designated urban area, remote from public transport infrastructure and other complementary facilities, it is considered the proposed development in this location would lack ancillary supporting services and would be reliant primarily on private transport.

"The proposed development would be inappropriately located, would contravene the policies as set out in the Regional Planning Guidelines for the Greater Dublin Area 2004-2016 to reduce the overall growth in demand for transport and would, therefore, be contrary to the proper planning and sustainable development of the area."

Mr Mansfield, who owns the luxury Citywest hotel complex which has become a favourite venue for political and sporting gatherings, has run foul of the planning authorities over a series of proposed developments.

South Dublin County Council took action against him over alleged unauthorised building work at his Weston air field near Lucan.

Enforcement notices for removal of a hangar structure were served on both Mr Mansfield and Weston Ltd. The council said buildings were rebuilt - in one case increasing in size by about 75pc.

Mr Mansfield strenuously denied that the airport work was unauthorised.

In a statement, Citywest Hotel, owned by Mr Mansfield, said the Bord Pleanala decision was "disappointing".

"We will look at our options when we see the full report from the Bord. A detailed statement will be issued at that stage, the statement added.

Green Party environment spokesperson Ciaran Cuffe TD said: "The right decision has been made. Whether people are building a conference centre or a home extension they should adhere to our planning laws instead of building first and only applying for planning retention afterwards."